1. Field of the Invention
The present invention relates to an image heating device that heats an image on a sheet.
2. Description of the Related Art
An image forming apparatus, such as a printer, a copying machine, a facsimile machine, or a multi-functional apparatus of these machines, which includes a fixing device (image heating device) of a belt heating type using a fixing belt (endless belt) has hitherto been put into practical use. In such a fixing device, a toner image, which is formed and born on a sheet (recording material) by an image forming method such as an electrophotographic process or an electrostatic recording process, is fixed on a surface of the recording material by heating.
Since such a fixing device of the belt heating type uses a thin fixing belt having low heat capacity and high thermal responsiveness, the temperature of the fixing belt can reach a fixing temperature in a short time from power-on. This greatly contributes to power saving of the image forming apparatus.
However, the thin fixing belt may be broken owing to deformation or a flaw caused by any external force. If the fixing belt is broken, an image defect may be caused. Therefore, if the fixing belt is broken, it is preferable to immediately understand the fact and to prohibit a fixing operation (image heating operation). A technique for that purpose is proposed in Japanese Patent Laid-Open No. 2002-287542.
Specifically, in a fixing device described in Japanese Patent Laid-Open No. 2002-287542, a belt mark is put on a fixing belt, and an optical sensor is disposed on a side opposed thereto. With this structure, it is determined that the fixing belt is broken when the optical sensor does not detect the belt mark for a fixed time.
However, in the fixing device described in Japanese Patent Laid-Open No. 2002-287542, if a flaw is made on the belt mark or a foreign substance adheres to the belt mark, the amount of light (amount of reflected light) received by the optical sensor decreases and becomes unstable. Hence, a breakage of the fixing belt may be detected erroneously.